An already nasty Miami-Dade County Commission campaign appears poised to get worse.

Candidate Juan C. Zapata is bracing for the latest attack, which he predicts will bring up a federal money-laundering conviction against his parents 30 years ago and his own brush with the law as a college student 20 years ago.

Political advertisements bringing up that history have yet to hit mailboxes or the airwaves. But Zapata, a former state representative, told The Miami Herald that he suspects a shadowy third-party group in Tallahassee that has already bankrolled several mailers against him is preparing to send out new ones as absentee ballots begin to arrive in voters’ homes.

“The strategy is to destroy Zapata,” he said. “ ‘Let’s point out the two or three blemishes on his record and go after him.’ ”

“I don’t know where they’re from, but obviously he may not have people who want to see him in office,” Machado said. “I have no reason to go negative.”

Machado decried two mailers against him that called him a “bureaucrat” and that suggested county unions are bankrolling his campaign. The fliers were sent by Citizens for Progress & Integrity, a local electioneering communications organization (ECO) that backs Zapata.

The two will face off in a nonpartisan Nov. 6 runoff to replace outgoing Commissioner Joe Martinez in District 11, which stretches across a wide swath of unincorporated West Miami-Dade.